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  #1  
Old Dec 25th, 12, 01:36 PM
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RJBaileyrn RJBaileyrn is offline
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RJ
 
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Default Jets and Carbs

Hi all,
Let's talk jetting and carbs for bit. Have been reading til I'm cross eyed about this.
My 406, with specs as in signature has what I would call a stumble when I am idling along at slow speed in lower gears, like you would be down a local street. It occurs when you attempt to ease down on the throttle and speed up just a tad. The motor seems like it doesn't want to accelerate at all until you give it even more gas, then it jerks ahead.

Jets are as I bought the Quick Fuel Drag 750 carb, 74s in front, 76s in back, 3.5 power valves front and back, idle vacuum-7.

Question is, is this stumble caused by not enough jetting in the primaries or what. Car is pretty streetable over all, just have this issue low down. With the Quik Fuel, I can adjust idle speed on both primaries and secondaries, but not sure how to proceed. Timing is set at 20 degrees initial and the rest all in by 3000, as I recall.

Opinions and suggestions appreciated.

RJ
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  #2  
Old Dec 25th, 12, 02:03 PM
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RJBaileyrn RJBaileyrn is offline
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RJ
 
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Default Re: Jets and Carbs

Another question. This carb, Quick Fuel Q Series 750, came with #74 primary jets, #84 jets with extensions, 4.5 primary power valve, secondary valve plugged. It is a race carb, has no choke, has mechanical secondaries. Question is, how does the factory determine these things. I assume it has to do with air and fuel flow testing. I read alot of guy's posts that say they used the carb right out of the box with no changes. How often is this applicable, given all the changes guys make to their engines.
Thanks
More to fololw haha
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  #3  
Old Dec 25th, 12, 02:34 PM
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Kevs67RSSS Kevs67RSSS is offline
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Default Re: Jets and Carbs

If its the jets I would think they just need to be cleaned and nothing more. Assuming here the car was running fine before. I wold also look/test the fuel pump (mechanical or electric) for operation. Also check the lines for any sort of blockage. Other than that it could be the idle circuit or the accelerator pump. I just don't think it has anything to do with the jets. When was the last time you had the carb rebuilt to include soaking and the idle circuits blown out with compressed air? Quickfuel carbs are good carbs to say the least. Your prob is just a matter of eliminating circuits and finding the bad one. My bet is on the idle or accelerator pump.
Hope this helps.
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  #4  
Old Dec 25th, 12, 02:43 PM
Eleanor's Nemesis Eleanor's Nemesis is offline
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Default Re: Jets and Carbs

My diagnosis would be that you either need to install a larger accellerator pump cam or install bigger squirters, or even both. It sounds like to me you are having a temporary overly lean condition until the proper amount of fuel comes on from the jets.

If you try to hammer it instead of rolling into the throttle does it fall flat on its face or even pop through the carb?

Quote:
Originally Posted by RJBaileyrn View Post
Hi all,
Let's talk jetting and carbs for bit. Have been reading til I'm cross eyed about this.
My 406, with specs as in signature has what I would call a stumble when I am idling along at slow speed in lower gears, like you would be down a local street. It occurs when you attempt to ease down on the throttle and speed up just a tad. The motor seems like it doesn't want to accelerate at all until you give it even more gas, then it jerks ahead.

Jets are as I bought the Quick Fuel Drag 750 carb, 74s in front, 76s in back, 3.5 power valves front and back, idle vacuum-7.

Question is, is this stumble caused by not enough jetting in the primaries or what. Car is pretty streetable over all, just have this issue low down. With the Quik Fuel, I can adjust idle speed on both primaries and secondaries, but not sure how to proceed. Timing is set at 20 degrees initial and the rest all in by 3000, as I recall.

Opinions and suggestions appreciated.

RJ
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1968 Street/Strip Camaro 350 w/PG
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  #5  
Old Dec 25th, 12, 02:45 PM
Eleanor's Nemesis Eleanor's Nemesis is offline
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Michael Gekko
 
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Default Re: Jets and Carbs

Virtually no carb will operate optimally 'out of the box'. They may work well, but the user is likely leaving a bit on the table. This is where some fine tuning will make a difference-sometimes very slight other times in a more dramatic fashion.

Quote:
Originally Posted by RJBaileyrn View Post
Another question. This carb, Quick Fuel Q Series 750, came with #74 primary jets, #84 jets with extensions, 4.5 primary power valve, secondary valve plugged. It is a race carb, has no choke, has mechanical secondaries. Question is, how does the factory determine these things. I assume it has to do with air and fuel flow testing. I read alot of guy's posts that say they used the carb right out of the box with no changes. How often is this applicable, given all the changes guys make to their engines.
Thanks
More to fololw haha
__________________
1968 Camaro NHRA/IHRA SS/C 'Super Stocker' 396/Jerico
1968 Street/Strip Camaro 350 w/PG
1968 Caprice NHRA/IHRA Stock ELiminator O/SA 327-250 HP



Land of the Free Because of the Brave

It Takes a Long Time to Grow Old Friends
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  #6  
Old Dec 25th, 12, 02:52 PM
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Default Re: Jets and Carbs

carb doesnt have more than 500 miles on it. fuel pump new, lines new, everything spotless, stumble is when you just ease into the throttle, not when you nail it. When you nail it car goes sideways in 1st three gears. Just annoying with the stumble in city driving.
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  #7  
Old Dec 25th, 12, 02:54 PM
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Default Re: Jets and Carbs

I guess where it is really obvious and annoying is when I am leaving traffic light or stop sign. Also cruise at 35-45 and 2500-3500 rpm. is rough feeling if that means anything to you
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Old Dec 25th, 12, 02:56 PM
Eleanor's Nemesis Eleanor's Nemesis is offline
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Michael Gekko
 
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Default Re: Jets and Carbs

Okay, have you checked for any vacuum leaks? Spray some carb cleaner around the base of the carb and the throttle shafts-when you do this listen to see if the rpm changes while it is idling, and keep a medium sized towel that is doused with water handy. Spray along any vacuum hoses too.

Are you using a vacuum advance on the distributor, and if you do are you using manifold vacuum to operate it?
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1968 Street/Strip Camaro 350 w/PG
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  #9  
Old Dec 25th, 12, 03:03 PM
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Default Re: Jets and Carbs

RJ - Carburetors are comprised of circuits or systems. Starting with the fuel inlet system, the idle system, transition, power etc... Each circuit has a function and you trouble shoot the part of the carb that pertains to the problem you are having.

Think about a car as a whole, it has a drive train, a suspension, brakes etc. You wouldn't trouble shoot the engine if you heard clunking coming from the front fender when you put on the brakes... Learn to recognize what part of the carb the symptom points to and you will become a much better tuner!

Get a Holley book like
Amazon.com: Holly Carburetor Handbook 4150 & 4160 Hp473 (0075478007509): Mike Urich: Books Amazon.com: Holly Carburetor Handbook 4150 & 4160 Hp473 (0075478007509): Mike Urich: Books


Jets are part of the power circuit and not likely to cause a part throttle stumble. Look at the opening on the top of your carb that lets air into the mixture. When you step on the gas far more air can rush into the carb than fuel so you have an accelerator pump (2 if it's a mechanical secondary) that operate and squirt extra fuel to compensate for the overpowering amount of air. This is part of the transition circuit along with the discharge nozzles and is most likely where the trouble is...
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  #10  
Old Dec 25th, 12, 03:08 PM
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RJBaileyrn RJBaileyrn is offline
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Default Re: Jets and Carbs

Appreciate the suggestions by the way
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  #11  
Old Dec 25th, 12, 03:09 PM
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RJBaileyrn RJBaileyrn is offline
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Default Re: Jets and Carbs

No vacuum advance. MSD 6AL, all mechanical
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  #12  
Old Dec 25th, 12, 03:11 PM
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RJBaileyrn RJBaileyrn is offline
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Default Re: Jets and Carbs

Slightest touch of the throttle linkage activates the squirters if that's where you are going Dennis. and thanks for the book recommendation.
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  #13  
Old Dec 25th, 12, 03:11 PM
Eleanor's Nemesis Eleanor's Nemesis is offline
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Michael Gekko
 
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Default Re: Jets and Carbs

Well that is why-or at least part of the reason why- you are having problems at part throttle or rolling into it. A distributor without a vacuum advance is designed really for either idling around the pits or WOT.

Can you find a distributor with a VA to try?
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1968 Camaro NHRA/IHRA SS/C 'Super Stocker' 396/Jerico
1968 Street/Strip Camaro 350 w/PG
1968 Caprice NHRA/IHRA Stock ELiminator O/SA 327-250 HP



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  #14  
Old Dec 25th, 12, 03:17 PM
Eleanor's Nemesis Eleanor's Nemesis is offline
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Michael Gekko
 
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Default Re: Jets and Carbs

Try reading this thread and post by John Z....some very good info here that may help you-

http://www.camaros.net/forums/showthread.php?t=150344
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1968 Camaro NHRA/IHRA SS/C 'Super Stocker' 396/Jerico
1968 Street/Strip Camaro 350 w/PG
1968 Caprice NHRA/IHRA Stock ELiminator O/SA 327-250 HP



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  #15  
Old Dec 25th, 12, 04:08 PM
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Default Re: Jets and Carbs

Quote:
Originally Posted by RJBaileyrn View Post
Slightest touch of the throttle linkage activates the squirters if that's where you are going Dennis. and thanks for the book recommendation.
That's a start but don't forget about volume and duration of the squirt. A simple accl pump cam position change or replacement can make a big difference, same with a smaller or larger nozzle and that just gets things going when you have 2 accl pumps...
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